Method of reducing warpage and dimensional growth of structural members during heat treatment



Oct. 11, 1966 A HUSEBY ETAL 3,278,349

METHOD OF REDUCING WARPAGE AND DIMENSIONAL GROWTH OF STRUCTURAL MEMBERSDURING HEAT TREATMENT Filed May 1, 1964 DIMENSIONAL CHANGE (TENTHOUSANDS OF AN INCH PER INCH) 5. -5 '0 5 I0 I I5 20 EROBERTJAWHMQJZENTOR DIMENSIONAL CHANGE "13? (TEN THOUSANDS OF AN INCH/mfru; FJ fiarle PER INCH) United States Fatent G 3,278,349 METHOD OFREDUCING WARIAGE AND DIMEN- SIGNAL GROWTH OF STRUCTURAL MEMBERS DURINGHEAT TREATMENT Robert A. Huseby and Edmond J. Nielsen, Milwaukee, Wis.,assignors to A. 0. Smith Corporation, Milwaukee, Wis, a corporation ofNew York Filed May 1, 1964, Ser. No. 364,119 5 Claims. (Cl. 148-431)This invention relates to metal processing and more particularly to amethod of minimizing warpage and dimensional growth in elongatedstructural members during heat treatment.

In the past, the quenching and subsequent straightening of long channelmembers, such as those used as truck trailer side rails, has been almosta completely manual operation. Channel members for truck trailer framesrange in length from 25 to 40 feet and are first heated and thenquenched and tempered to increase the tensile strength and hardness ofthe frame. The payload which may be carried under the gross weight limitlaws for State and Federal highways is thereby increased because alighter, stronger frame results. The heat treatment causes considerablewarpage, such as twisting, bowling and cambering, and the warpage mustbe removed before the channel member is acceptable for use as a truck ortruck trailer side rail. In the past, the channel members were heated,quenched and tempered and then cold straightened manually with the aidof hydraulic presses, and primitive straightening bars to remove allwarpage from the channel member.

More recently, it has been proposed to die-quench the channel members inorder to reduce the warpage. In the die-quenching operation, the channelmember is taken from the furnace and held in a die during the quenchingoperation. This die-quenching operation substantially reduces thewarpage in the long channel member, and therefore minimizes the amountof manual straightening that is required. While the die-quenchingoperation eliminates a substantial amount of the warpage, the die cannothold the long workpiece against all warpage. If a suificient force isexerted to hold the long channel member against all warpage, cracking ofthe member results. In addition, even when using the die-quenchingoperation, dimensional growth of the channel is produced and dimensionalgrowth is undesirable.

During the blanking operation, the flat metal blank is not only cut tosize, but is also provided with a series of holes which are used forconnections to other structural members in the vehicle frame. In someside rails for trucks and truck trailers, the blank may be provided withup to 300 holes. During the heat treating operation, a variation indimensional growth occurs. This dimensional growth may varysubstantially from piece-to-piece so that the holes are not properlyaligned when the channel members are assembled into the frame, and inmany cases the holes must be re-drilled in order to assemble the frame.

The present invention is directed to a method of minimizing dimensionalgrowth and warpage in elongated structural members during heattreatment. According to the invention, the elongated structure member,such as a channel or angle, is initially heated to -a temperature abovethe upper critical temperature, and .then subjected to a two-stepcooling treatment. The member is initially cooled from a temperatureabove the upper critical temperature to a temperature below the uppercritical temperature in the range of 1300 F. to 1650 F. and is thensubjected to a die-quenching treatment which reduces the temperature tobelow 800 F.

Patented Oct. 11, 1966 ICC This two-step cooling treatment for theelongated structural member minimizes warpage, including bowing,cambering and twisting and thereby substantially eliminates theexpensive hand-straightening operation which was necessary with priorheat treating methods. In addition, the treatment of the inventionreduces the dimensional growth of the member and thus makes assembly ofthe frame easier and eliminates the re-drilling of holes.

Other objects and advantages will appear in the course of the followingdescription.

In the drawings:

FIG. 1 is a normal frequency curve illustrating the variation indimensional growth of carbon steel channels subjected to a single stepdie-quenching operation; and

FIG. 2 is a curve similar to FIG. 1 showing the reduced dimensionalgrowth in a carbon steel channel subjected to the two-step die-quenchingoperation of the invention.

The structural member which is to be subjected to the heat treatment ofthe invention is generally a channel or angle having a thickness lessthan /2 inch and having a substantial length usually in the neighborhoodof 20 to 40 feet. The invention is particularly adapted to the heattreatment of carbon steel truck side rails which are channel shaped incross section, having a length of 25 to 40 feet, and a thickness of Ainch to inch.

The steel structural member to be treated has a total alloy content lessthan 3% by weight and has a carbon content less than 0.30% and generallyin the range of 0.15 to 0.30%. In addition, the member may contain 0.85to 1.50% manganese. Phosphorous should be maintained less than 0.04%,sulfur should be maintained below 0.05% The steel structural member isinitially heated to a temperature above the upper critical temperatureand generally to a temperature in the range of 25 to 50 above the uppercritical temperature. For most alloys, this temperature will be in therange of 1500 F. to 1900 F. Heating to this temperature can be at anydesired rate and the rate of heating is not critical The member is heldat this temperature for a period of time sufficient to obtain a uniformtemperature and carbon distribution throughout the piece. Generally, themember will be held at the temperature for a period of time of 2 to 15minutes.

Following the holding temperature, the member is cooled to a temperaturebelow the upper critical temperature and generally to a temperature inthe range of 1300 to 1650 F. The cooling rate to the temperature of 1300F. to 1650 F. is not critical and the member can be cooled to thistemperature range by furnace cooling, atmosphere cooling or by quenchingin a salt bath or the like. Cooling to the range of 1300 F. to 1650 F.should be accomplished in less than 30 minutes for most articles andusually less than 15 minutes.

Members having a carbon content in the lower portion of theaforementioned carbon range are usually cooled to a temperature in theupper portion of the l300 to 1650 F. range, while conversely, membershaving a carbon content in the upper portion of the aforementionedcarbon range are usually cooled to a temperature in the lower portion ofthe l300 to 1650" F. range.

The member is held at the temperature of 1300 to 1650 F. for a period oftime sufficient to get a uniform distribution of temperature. Thisperiod is generally in the range of about 3 to 15 minutes.

The member is then inserted into a die and quenched to a temperaturebelow 800 F. at a rate faster than 400 F. per second, and usually in therange of 400 F. to 1600 F. per second, by using oil, water or the like.The member can be die-quenched with the apparatus disclosed in thecopending application of Gerald J. Kuchera, Serial No. 73,954, filedDecember 5, 1960 and entitled Method and Apparatus for Quenching a MetalMember. As disclosed in this prior application, the die-quenchingapparatus comprises expandable male and female dies and the channelmember is inserted between the dies. Both sets of dies have a network ofgrooves which gives the die surfaces a waflle-like appearance, and thedies have internal passages which communicate with the wattle-likesurfaces. A cooling medium, such as water, is supplied through thepassages to the wattle-like surfaces of both the male and female diemembers which are disposed in contact with the channel member.

In operation, the heated channel member is transferred web down into thefemale dies of the die sections. The movable side wall of the female dieis then moved in, clamping the hot channel member between fixed andmoveable side walls of the female die sections thereby straightening thechannel member. The expandable male dies are then moved toward thefemale die members and into the channel member throughout its length andextended laterally into engagement with the side flanges of the channelmember. Quenching water is then circulated under pressure through thecooling system and the cooling water flows abundantly through thenetwork of intersecting grooves in the male and female die surfaces andinto direct contact with all surfaces of the hot channel member. Thecooling water quenches the channel member rapidly and uniformly while itis being held by the die assemblies. The male dies are then withdrawnfrom the quenched channel member, the female dies expanded, and anunloader mechanism ejects the channel member from the quench machine.

The channel member, after die quenching, can be cooled to roomtemperature in any desired rate, and the channel member is subsequentlysubjected to a stress relieving treatment to relieve any residualstresses in the member. The stress relieving treatment generallyconsists of reheating the member to a temperature up to 1100 F. andusually in the range of 600 to 1000 F. for a period of up to 30 minutes,depending on the properties desired.

FIGS. 12 illustrate the reduction in the variation of dimensional growthbrought out by the process of the invention. The curves shown in FIGS.1-2 are normal frequency distribution curves made in accordance with theformulae shown in Analysis and Correlation of Test Data Irving W. Burr,Metals Progress September 1952. FIG. 1 shows the dimensional variationsof carbon steel (A. O. Smith 112A, which has the followingspecification: 0.22-0.27% carbon, 0.851.20% manganese, 0.04% max.phosphorus 0.05% max. sulfur, balance iron) channels having a thicknessof inch and a length of 30 feet when subjected to a single step quench.The channels were initially heated to 1750 F., held as this tem peraturefor 22 minutes and then die quenched with water to 500 F. in 1.2seconds. The channels were subsequently stress relieved by heating in afurnace to 900 F. with a total cycle time of 30 minutes.

FIG. 1 shows that 272 pieces were subjected to this treatment and theaverage dimensional growth (shown by the peak of the curve) was 0.0009inch per inch. The spread of dimensional change was from 0.0004 inch perinch (indicating a shrinkage) to +0.0022 inch per inch, or a totalspread or variation of 0.0026 inch per inch.

FIG. 2 illustrates a similar type curve for channels which weresubjected to the two-step cooling procedure of the invention. Thechannels were of the identical size and composition to those tested inFIG. 1 and were initially heated to 1750 F. and held at this temperaturefor 3 minutes. The channels were then furnace cooled to 1450" F. in 5minutes, held at this temperature for 4 minutes, and then die quenchedwith water to 450 F. in 1.1 seconds. The die-quenching apparatus wassimilar to that previously described. The channels were subsequentlystress relieved by heating in a furnace 900 F. with a total cycle timeof 30 minutes.

FIG. 2 shows that 20 pieces were subjected to this two-step coolingtreatment and the average dimensional growth was 0.0006 inch per inchconsiderably less than the average growth which occurred with thesingle-step quench, as shown in FIG. 1. More important, was thereduction in variation in dimensional change brought about by thetwo-step quench. In FIG. 2, the spread of dimensional change was from-0.0001 inch per inch to +0.0013 inch per inch, or a total variation of0.0014 inch per inch, as compared with a variation of 0.0026 inch perinch with the single-step quench shown in FIG. 1.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

We claim:

1. A method of heat treating elongated structural steel members tominimize warpage and dimensional growth, comprising the steps of heatinga structural steel member having a carbon content of less than 0.30% toa temperature above the upper critical temperature, holding the memberat said temperature for a period of time sufficient to obtain a uniformdistribution of temperature, cooling the member to a temperature in therange of 1300 to 1650 F., holding the member at said second namedtemperature for a period of time sufiicient to obtain a uniformdistribution of temperature, and thereafter die quenching the member toa temperature below 800 F.

2. A method of heat treating elongated structural steel members tominimize warpage and dimensional growth, comprising the steps of heatinga structural steel member having a carbon content of less than 0.30% toa temperature above the upper critical temperature, holding the memberat said temperature for a period of time sufficient to obtain a uniformdistribution of temperature, cooling the member to a temperature in therange of l300 to 1650 F., holding the member at said second named temperature for a period of time sufiicient to obtain a uniformdistribution of temperature, positioning the member within a die, andquenching the member while held in the die to a temperature below 800 F.

3. A method of heat treating elongated structural steel members tominimize warpage and dimensional growth, comprising the steps of heatinga structural steel member consisting essentially of .15 to 30% carbon,.85 to 1.50% manganese and balance iron to a temperature in the range of1600 to 1900 F., holding the member at said temperature for a period oftime sutficient to obtain a uniform distribution of temperature, coolingthe member to a temperature in the range of 1300 to 1650 F., holding themember at said last named temperature for a period suflicient to obtaina uniform distribution of temperature, and thereafter die quenching themember to a temperature below 800 F. at a rate of 400 F. to 1600 F. persecond.

4. A method of forming an elongated vehicle frame side rail comprisingthe steps of forming a flat steel blank having a carbon content lessthan 0.30% into a channel member having a U-shaped cross section,heating the channel member to a temperature in the range of 1600 to 1900F., holding the channel member at said temperature for a period of timesuflicient to obtain a uniform distribution of temperature, cooling thechannel member to a temperature in the range of 1300 to 1500 F., holdingthe channel member at said last named temperature for a period of timejust sufficient to obtain a uniform distribution of temperature,clamping the channel member in a die while at said last namedtemperature to straighten the channel member, and thereafter water 5quenching the channel member to a temperature below 800 F.

5. A method of forming an elongated vehicle frame side rail comprisingthe steps of punching a series of connecting holes in a steel blankhaving a total alloy content less than 3% by weight and having a carboncontent less than 0.30% and a thickness less than /2 inch, forming theblank into a channel member, heating the channel member to a temperaturein the range of 1600 to 1900 F., holding the channel member at saidtemperature for a period of time sufiicient to obtain a uniformdistribution of temperature, cooling the channel member to a temperaturein the range of 1300 to 1650 F., holding the channel member at said lastnamed temperature for a period of time just sufficient to obtain auniform distri- References Cited by the Examiner UNITED STATES PATENTS9/1964 Williams et al. 1483l DAVID L. RECK, Primary Examiner.

) C. N. LOVELL, Assistant Examiner.

3. A METHOD OF HEAT TREATING ELONGATED STRUCTURAL STEEL MEMBERS TOMINIMIZE WARPAGE AND DIMENSIONAL GROWTH COMPRISING THE STEPS OF HEATINGA STRUCTURAL STEEL MEMBER CONSISTING ESSENTIALLY OF .15 TO 30% CARBON,.85 TO 1.50% MANGANESE AND BALANCE IRON TO A TEMPERATURE IN THE RANGE OF1600* TO 1900* F., HOLDING THE MEMBER AT SAID TEMPERATURE FOR A PERIODOF TIME SUFFICIENT TO OBTAIN A UNIFORM DISTRIBUTION OF TEMPERATURE,COOLING THE MEMBER TO A TEMPERATURE IN THE RANGE OF 1300* C TO 1650* F.,HOLDNG THE MEMBER AT SAID LAST NAMED TEMPERATURE FOR A PERIOD SUFFICIENTTO OBTAIN A UNIFORM DISTRIBUTION OF TEMPERATURE, AND THEREAFTER THEQUENCHING THE MEMBER TO A TEMPERATURE BELOW 800* F. AT A RATE OF 400* F.TO 1600* F. PER SECOND.